Network dissolution effects, leaching

Figure 2. Effect of various stages of corrosion on changes of glass surfaces. Conditions (b) type 2, a = 0, selective leaching (c) type 4, 0 < a < 1, selective leaching and network dissolution and (d) type 5, a = 1, network dissolution. (Reproduced, with permission, from Ref. 7. Copyright 1980, North-Holland Publishing...

Waste form leach rates in a geologic repository will be affected by unknown water flow rates and by extensive cracking of the waste form monolith. An understanding of these effects is important in predicting the geochemical behavior of disposed radioactive waste forms over the full range of possible scenarios. The dependence of the waste form source term on the rate of renewal of aqueous solution is first established for the simple but important case of solubility-limited network dissolution control. 

In the present paper, flow rate effects on leaching are analyzed from a theoretical point of view and are rationalized in a consistent, generic leaching model incorporating the dependence of the mechanisms of selective leaching and network dissolution on solution feedback effects. 

A distinguishing effect of flow rate is that saturation of the solution is never achieved under dynamic leaching conditions. Thus, network dissolution, a solubility limited process, is never allowed to halt. A steady-state condition is eventually established whereby leach rates of network formers equalize the rate of removal of species from the leachant due to the flowing solution. This can be illustrated by formulating a network dissolution model which incorporates the dependence of leaching of network formers on solubility limits and water flow rates. 

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